The White House has denied an assertion by North Korea's foreign minister that the United States declared war on it, calling the claim "absurd."

Spokeswoman Sarah Sanders made the statement to reporters just hours after North Korea's top diplomat asserted Pyongyang's right to defend itself, and to shoot down U.S. strategic bombers.

"We've not declared war on North Korea. Frankly, the suggestion of that is absurd," Sanders said on September 25.

Speaking in New York, where he was attending the United Nations General Assembly, Ri Yong Ho told reporters that "all options are on the table" for North Korea as it responded to United States.

"The whole world should clearly remember it was the U.S. who first declared war on our country," Ri said, speaking through a translator.

On September 23, U.S. B-1B Lancer bombers, escorted by fighter jets, flew in international airspace over waters east of North Korea, the first time in decades that U.S military jets have flown in such proximity to the peninsula.

Ri warned that Pyongyang had the right to shoot down such bombers even if they were outside North Korean airspace.

Trump last week called North Korean leader Kim Jong Un a "madman," a day after Kim dubbed the U.S. president a "mentally deranged U.S. dotard."